Messi had played just 37 minutes for his club since September 3, having returned with an injury from international duty last month.
"We still have one more training session and one more is important for him... I have to talk it over with him and make sure we are convinced he can start. He looked good. Based on that, we will see the rest of the team," said Scaloni.
More than just Paraguay to consider
The Argentina coach said he also had to take into account that his team would face Peru in Lima five days later.
"What leaves us calm is that, if he is not fit to play, whoever goes out will do just as well, that's the most important thing," he said.
Argentina have won both their qualifiers so far and are level on six points at the top of the table with Brazil.
"They've already played together as starters. I'm open to many things. It will depend on the match. It wouldn't be a problem. They're different, but they can play," he added.
Paraguay have one point from their opening games and sit in sixth place, the final spot for automatic qualification for the 2026 World Cup.
Brazil host Venezuela in Cuiaba on Thursday -- the 'Vinotinto' have three points after a 1-0 win over Paraguay and a narrow 1-0 loss to Colombia.
Midfielder Casemiro says the 'Selecao' won't underestimate an opponent looking for their first appearance in the finals.
No easy games
"It's going to be a difficult game. Venezuela have their merits, many players playing in Europe, quality players. The important thing here is respect," the Manchester United and former Real Madrid midfielder told a news conference.
"Football is one of the few sports in which the favourite does not always win... and in South America there's no easy game," he added.
Brazil will be playing their third game under interim coach Fernando Diniz and Casemiro said the team are still adjusting to a new approach.
"We are trying to adapt as quickly as possible, but it takes time. At club level it usually takes months. In the national team we are trying to do it with videos and so on to correspond to that work philosophy as quickly as possible and minimise mistakes," he said.
After the clash with Venezuela, Brazil visit Marcelo Bielsa's Uruguay on Tuesday in Montevideo. Venezuela will host Chile.
Thursday's other games see Colombia host Uruguay, Bolivia take on Ecuador and Peru visit Chile.